EP0 240 732-A ("EP 732-A") discloses a process for welding butt-ended solid wall tubes of plastic. The entire end surfaces of the tubes to be welded are heated by a welding element applied between the tubes, thereby producing a undesirable welding rim on the inner circumference of the tubes. In order to keep the tubes to be joined and the welding element in alignment during welding, a special saddle part which can be placed on the outer circumference of the tubes is necessary. For compound tubes (also called double wall tubes) used recently for sewerage, for instance, the welded joint process disclosed in EP-732-A is not suitable. Tubes of this type comprise a thin-walled cylindrical inner tube and a corrugated outer tube firmly connected to the inner tube.
IT is therefore an object of the present invention to provide an apparatus and method by means of which a centred welded joint of tubular parts can be produced, without welding rims occurring on their inner circumference. Another object of this invention is to provide such an apparatus and method which should be suitable for the above mentioned compound tubes.